More particularly, the invention relates to a method of decoding a multivalent electrical signal capable of taking up a number n of states greater than 2, said signal having a characteristic electrical magnitude U which varies over time to take up successive values each representing one of the n states of the electrical signal, the characteristic electrical magnitude substantially retaining said successive values during pause periods each of predetermined constant duration T, the method including the following steps:
a) measuring a value of the characteristic electrical magnitude of the signal during each pause period; and PA1 b) identifying the state represented by each measured value. PA1 a the various thresholds S.sub.1, S.sub.2, . . . , S.sub.n-1 are determined not only as a function of the measured values U.sub.1, U.sub.2, . . . , U.sub.n, but also as a function of ideal values U.sub.1 0, U.sub.2 0, . . . , U.sub.n 0 that the characteristic electrical magnitude U ought to take for the various states of the signal; PA1 the various thresholds are given by: ##EQU1## where k is an integer lying in the range 1 to n-1, U.sub.1 0 being the smallest of the above-mentioned ideal values, U.sub.n 0 being the largest of the above-mentioned ideal values, and U.sub.k 0 and U.sub.k+1 0 being respectively the k-th and the (k+1)-th of said ideal values in increasing order; PA1 which values U.sub.1, U.sub.2, . . . U.sub.n of the characteristic electrical magnitude U corresponding respectively to the n states of the signal are measured during the adaptation period, with the various thresholds then being given by: ##EQU2## where k is an integer number lying in the range 1 to n-1, U.sub.k and U.sub.k+1 being respectively the k-th and the (k+1)-th of said measured values in increasing order; PA1 the various thresholds are given by: ##EQU3## where k is an integer lying in the range 1 to n-1; the n-1 thresholds S.sub.1, S.sub.2, . . . , S.sub.n-1 are regularly updated during decoding of the signal, by determining at least the minimum value U.sub.1 and the maximum value U.sub.n of the characteristic electrical magnitude during successive adaptation periods; PA1 while calculating the thresholds S.sub.1, S.sub.2, . . . , S.sub.n-1, the minimum value U.sub.1 and the maximum value U.sub.n as determined during the adaptation period are compared respectively with first and second ranges of predetermined values, the minimum value being rejected if it does not lie within the first range of values and the maximum value being rejected if it does not lie within the second range of values; PA1 the n-1 thresholds include a minimum threshold S.sub.1 and a maximum threshold S.sub.n-1, and during step b), each measured value of the characteristic electrical magnitude of the signal is compared firstly with a value S.sub.1 -.DELTA. and secondly with a value S.sub.n-1 +.DELTA., .DELTA. being a predetermined value, the measured value not being taken into account if it is less than S.sub.1 -.DELTA. or greater than S.sub.n-1 +.DELTA.; PA1 the value .DELTA. represent a constant fraction of the difference between the maximum threshold S.sub.n-1 and the minimum threshold S.sub.1 ; PA1 m is an integer not less than 7; and PA1 the electrical signal is the result of demodulating a modulated radio signal. PA1 a) measure a value of the characteristic electrical magnitude of the signal during each pause period; and PA1 b) identify the state represented by each measured value; the apparatus being characterized in that the decoder means measure the electrical magnitude at instants that are separated from a time origin by an integer number of durations T, said decoder means being designed to determine the time origin by performing close-together measurements of said characteristic electrical magnitude during a predetermined synchronization period at instants which are spaced apart from one another by a duration T/m, where m is an integer not less than 3, the decoder means also being designed to use said close-together measurements to identify the pause periods of the characteristic electrical magnitude U, and the decoder means finally being designed to set the time origin substantially in the middle of one of said pause periods. PA1 the decoder means are designed to identify the state represented by each measured value by comparing said measured value with n-1 thresholds S.sub.1, S.sub.2, . . . , S.sub.n-1 defining n ranges of values each corresponding to one state of the electrical signal, the decoder means also being designed initially to set said thresholds at least as a function of a minimum value U.sub.1 and of a maximum value U.sub.n of the characteristic electrical magnitude U as measured during a predetermined adaptation period; and PA1 the decoder means are designed during said adaptation period to measure the values U.sub.1, U.sub.2, . . . , U.sub.n of the characteristic electrical magnitude corresponding respectively to the n states of the signal, and to calculate the various thresholds using the following formula: ##EQU4## where k is an integer lying in the range 1 to n-1, and the apparatus further includes a non-volatile memory in which said various values U.sub.1, U.sub.2, . . . , U.sub.n measured during the adaptation period are stored.
In such a method, it is difficult to choose the instant at which to perform each measurement. A measurement performed too near the beginning or the end of a pause can be falsified by the fact that the characteristic electrical magnitude does not remain exactly constant over the entire pause. Under such circumstances, identification of the state represented by the measured value can itself be falsified.
A particular object of the present invention is to mitigate these drawbacks.
To this end, according to the invention, a method of the kind in question is essentially characterized in that the characteristic electrical magnitude U is measured at instants that are separated from a time origin by an integer number of durations T, the time origin being determined by performing close-together measurements of said characteristic electrical magnitude during a predetermined synchronization period at instants that are spaced apart from one another by a duration T/m, where m is an integer not less than 3, said close-together measurements being used to identify the pause periods in the characteristic electrical magnitude U, with the time origin being set substantially in the middle of one of said pause periods.
In addition, unlike decoding a bivalent signal where it is easy to discriminate between a high level and a low level of the signal, identification of the state represented by each measured value can be disturbed relatively easily by variations in said characteristic electrical magnitude, which variations may be due, in particular to temperature variations of the electronic circuits processing the signal, and to aging of these circuits.
Also, when the decoding method is applied to an electrical signal that is the result of demodulating a previously picked-up radio signal, e.g. when the radio signal is modulated in frequency, in amplitude, or in phase, the disturbing variations in the characteristic electrical magnitude can also be due to the better or worse quality of reception of the radio signal.